1. Field of the Invention
The present invention relates generally to the field of valves, and more specifically, to a reciprocating stemmed valve that incorporates a planetary gear system and prevents leakage of fluid to the atmosphere.
2. Description of the Related Art
A number of patent applications have been filed for valve actuators that mitigate stem leakage through the use of a magnetic interlock. These actuator chambers either enclose the dynamic seal that is present in every valve around the stem of the valves, or they eliminate the need for the seal entirely. This dynamic seal is known as a packing or mechanical seal. The magnetic interlock is employed to transmit force from outside of the actuator chamber to the inside, thus avoiding the penetration of the chamber wall by a mechanical stem actuator. Penetration of the chamber wall would nullify the purpose for the chamber in the first place—to enclose the dynamic seal around the stem and prevent leakage from the seal.
The problem with the various magnetic actuators proposed is that the amount of force transmitted by the magnets is not adequate to ensure the proper function of the valve. If an actuator is designed to provide adequate force to open and close the valve, the magnet coupling is so large as to make it impractical. Even with the use of modern rare-earth magnets such as Neodymium-Iron-Boron and Samarium-Cobalt, the ability to transmit adequate force to the valve stem is still difficult. The forces provided by the magnets are only a fraction (usually less than 20%) of the force that a mechanical stem actuator can provide. This does not give the valve operator the confidence that his valve can be opened or closed under situations where high force is required, such as high fluid pressure, dry seals, or debris in the fluid path.
Rather than increasing force by building ever larger magnetic couplings, the present invention incorporates a set of planetary gears to take the force supplied by the inner magnetic coupling and magnify it many times over through gear speed reduction (i.e., the use of reducing gears). For example, through the use of a planetary gear subassembly, the rotational movement supplied by the inner magnetic cartridge is reduced three-fold, while at the same time the force supplied by the inner magnetic cartridge is magnified three-fold. This means that by using a planetary gear assembly with a 3:1 ratio (i.e., the outer magnetic cartridge rotates three times for every one rotation of the internal thread ring), one can either gain three times as much force for the valve stem, or else the size of the magnetic coupling can be reduced by three times. The reduction in size is desirable because the magnetic coupling is the most expensive component of the actuator, and its size is generally proportional to its cost.
Through the incorporation of a planetary gear subassembly, the present invention provides a magnetically activated valve actuator that can be used in the harshest conditions. Magnetic actuation is no longer appropriate for light applications only. Rather, it is a robust alternative that provides force to the stem that is equivalent to that of dynamically sealed stemmed valves. This innovation is most needed in places like chemical plants, refineries, paint factories, paper mills, etc. where valves are the central workhorses of the plant itself.
In addition to increasing force and/or decreasing the size of the magnetic coupling, the present invention has the advantage of completely containing any leakage of fluids from the valve bonnet. The present invention is intended to be coupled to valves that are used in hazardous fluid or chemical applications, where stem leakage poses a pollution threat to the outside environment or a safety threat to personnel working nearby. At the very least, leakage from stem packings results in the loss of product, which can be costly. Fugitive emissions account for over 125,000 metric tones of lost product per year in the United States alone. Of this amount, the percentage of fugitive emissions that come from valve stems is estimated to be between 60% and 85%. [1, 2]
The threat posed to the environment by leaking valve stems is great, particularly when the product that is leaked is a fugitive emission, that is, a leaked or spilled product that cannot be collected back from the environment. An example of a fugitive emission would be methane leaking from a valve on a pipeline or in a refinery, in which case the methane immediately goes into the atmosphere and cannot be recaptured. Another example would be crude oil leakage from a valve on an offshore rig, where the oil is carried away by ocean currents and cannot be brought back.
Safety requirements are becoming more stringent with each passing year. Personnel who are required to work near hazardous chemicals—such as operators in a petrochemical plant—are subject to injury from leaking valve stems, especially from reciprocating stems where the hazardous material inside the valve is transported to the outside environment via the stem as it retracts from the valve body. For example, if the valve is handling chlorine, a leaking stem transports it to the outside environment, where it becomes hydrochloric acid when it reacts with moisture in the air. This acid corrodes the stem, which makes it even more difficult to seal as time goes by.
The above examples illustrate the need for leak-free valves. The magnetic actuator of the present invention, described more fully below, is capable of addressing this need by safely enclosing the dynamic (stem) seal of reciprocating stemmed valves.